The present invention relates to integrated circuit structures and fabrication methods.
Titanium nitride (TiN) is commonly used as a diffusion barrier in contacts, vias, and trenches and in interconnect stacks. It also may serve as a "glue layer" for chemical vapor deposited (CVD) tungsten and a nucleation layer for CVD tungsten and CVD aluminum. A good barrier layer should have: good step coverage to achieve void-free plug formation and adequate barrier thickness at the bottom of the contact/via/trench; good diffusion barrier properties to prevent diffusion of metals and WF.sub.6 attack of the underlying metal or silicon during tungsten deposition; inertness and low reactivity with adjacent materials during thermal cycles; and acceptable electrical properties such as low resistivity, low contact/via resistance, and low junction leakage. In addition, the barrier layer should be as thin as possible so as to reduce the interconnect stack thickness.
Traditionally, reactively sputtered TiN and TiN formed by rapid thermal nitridation of titanium have been used as diffusion barriers. Presently, there is a trend to replace PVD TiN by CVD TiN to meet the step coverage requirements for sub 0.35 .mu.m contacts, trenches, and vias. CVD TiN overcomes the metal reliability problems and the junction leakage issues associated with PVD TiN. CVD TiN can withstand thermal stresses of 550.degree. C. while maintaining low contact resistance and leakage. In addition, CVD TiN is potentially a cleaner process than collimated PVD TiN.
CVD TiN deposited by reacting TiCl.sub.4 with NH.sub.3 has been utilized but has several problems. Some of these problems are: high deposition temperatures, chlorine incorporation as TiNCl, and formation of NH.sub.4 Cl particles in the gas phase. While chlorine contamination can be reduced, it can not be eliminated using such a process. If the film must be deposited at a temperature lower than 400.degree. C., the TiCl.sub.4 /NH.sub.3 process can not be utilized but a metal-organic precursor may be used. Two commonly used metal-organic precursors are tetrakisdimethylaminotitanium (TDMAT) and tetrakisdiethylaminotitanium (TDEAT). Deposition of CVD TiN by the thermal decomposition of TDMAT results in a layer with good step coverage and low particle counts but results in unstable films with high resistivity. Resistivity can be improved by reacting the metal-organic precursor with ammonia. However, a reaction of TDMAT with NH.sub.3 results in a film with poor step coverage and has potential problems associated with gas-phase reactions, such as particle formation.
The literature has discussed the possibility of using TDMAT or TDEAT without ammonia to deposit TiN, but teaches AWAY from this possibility: films deposited from TDEAT or TDMAT without ammonia are said to have very poor properties. See e.g. Sun and Tsai, "Characterization of low pressure chemical-vapor-deposited titanium nitride from metalorganic sources," in ESSDERC '94 PROCEEDINGS at pp.291-4, which is hereby incorporated by reference.
Ti--Si--N compounds provide an even better diffusion barrier than TiN, and hence are attractive for advanced metallization applications. Currently, two major methods are being explored for preparing Ti--Si--N based films; however, both have significant limitations. Reactive sputtering (of Ti--Si targets in an N.sub.2 ambient) is the most established method, represented by extensive studies at Cal Tech. However, due to the directional nature of the sputtering method, the step coverage of deposited films is very poor for high aspect ratio contacts, vias, and trenches.
A chemical vapor deposition process using a mixture of silane, ammonia, and TDEAT is being investigated at Sandia National Lab. This method can provide films with better step coverage; however, the gas phase reaction between TDEAT and NH.sub.3 results in particulate formation, and the films produced have high defect density.
Innovative Structures and Methods
The present application discloses innovative processes for fabricating conformal Ti--Si--N films with low defect density, which solves the problems encountered by both reported methods. These processes first deposit a porous barrier layer containing titanium and nitrogen, and thereafter perform a postdeposition treatment which introduces silicon into at least the upper surface of the porous layer, to provide a silicon-rich surface. (In an alternative embodiment, the postdeposition treatment introduces boron instead of or in addition to silicon.) The porous barrier layer preferably also includes a significant fraction of carbon, and optionally also includes a significant fraction of silicon before the postdeposition treatment.
Advantages of the disclosed methods and structures include:
The Si-rich surface of the Ti--Si--N film (or B-rich surface of Ti--B--N film) minimizes the absorption of oxygen into the films, and therefore stabilizes the resulting films. PA0 The Si-rich or B-rich surfaces are also helpful in wetting Al and enhancing adhesion to Cu, and therefore are advantageous for advanced metallization application. PA0 Compared with the sputtering method, this invention offers a process for depositing films with much better step coverage and easier control of Si/Ti ratio. PA0 Compared with the TDEAT+NH.sub.3 +SiH.sub.4 method, this invention eliminates the gas phase reaction between Ti source and NH.sub.3. PA0 The disclosed process is capable of providing conformal films with low defect density. PA0 The disclosed process is flexible in controlling chemical composition, including surface composition. PA0 The disclosed process can be performed in commercial CVD reactors and can be easily implemented.